Stress distribution tuning in composite plugs and method

ABSTRACT

A composite plug for sealing a well, the composite plug including a mandrel having an internal bore that extends along a longitudinal axis (X) of the mandrel; and plural elements located on the mandrel. At least one element of the plural elements is attached to the mandrel with a tunable thread, and the mandrel and the tunable thread are made of composite materials.

BACKGROUND TECHNICAL FIELD

Embodiments of the subject matter disclosed herein generally relate to downhole tools related to perforating and/or fracturing operations, and more specifically, to a composite plug in which one or more parts has threads manufactured to achieve a desired stress distribution.

DISCUSSION OF THE BACKGROUND

In the oil and gas field, after a well 100 is drilled to a desired depth H relative to the surface 110, as illustrated in FIG. 1, and the casing 102 protecting the wellbore 104 has been installed and cemented in place, it is time to connect the wellbore 104 to the subterranean formations 106 to extract the oil and/or gas. This process of connecting the wellbore to the subterranean formations may include a step of plugging the well with a plug 112 and a step of making holes 116 into the casing.

The step of plugging the well requires lowering into the well 100 a setting tool 120, which is mechanically connected to a perforating gun assembly 114, which in turn is attached to a wireline 118. The setting tool is configured to set the plug at the desired location. Setting tool 120 is configured to hold the plug 112 prior to plugging the well. FIG. 1 shows the setting tool 120 disconnected from the plug 112, indicating that the plug has been set in the casing and the setting tool 120 has been disconnected from the plug 112.

FIG. 1 shows the wireline 118, which includes at least one electrical connector, being connected to a control interface 122, located on the ground 110, above the well 100. An operator of the control interface (e.g., a computer) may send electrical signals to the setting tool for (1) setting the plug 112 and (2) disconnecting the setting tool from the plug. After the plug has been set and the holes 116 in the casing have been made, the setting tool 120 is taken out of the well and a ball 122 is typically inserted into the well to fully close the plug 112. When the plug is closed, a fluid 124, (e.g., water, water and sand, fracturing fluid, etc.) may be pumped by a pumping system 126, down the well for fracturing purposes.

The above operations may be repeated multiple times for perforating the casing at multiple locations, and fracturing different stages associated with underground formations 108 and 109. Note that in this case, multiple plugs 112 and 112′ may be used for isolating the respective stages from each other during the perforating phase and/or fracturing phase.

A plug 200 has, as shown in FIG. 2, an internal bore 202 that allows a fluid to pass through the plug. FIG. 2 also shows the other components of the plug, i.e., a mandrel 204, a push ring 206, an upper slip ring 208, an upper wedge 210, a sealing element 212, a lower wedge 214, a lower slip ring 216, and a mule shoe 218. The mandrel 204 supports all these components. The push ring 206, when pressed by the setting tool (or a setting kit), moves the upper wedge 210 under the upper slip ring 208, thus breaking the upper slip ring 208 and pressing its various parts against the casing. The same action happens with the lower slip ring 216 and the lower wedge 214. The sealing element 212 is pressed between the two wedges, thus expanding radially and sealing the well. The mule shoe 218 acts as a reaction component during the setting of the plug. This means that while the push ring is pushing all the elements discussed above toward the mule shoe 218, the mule shoe prevents these elements from being pushed out of the mandrel. Thus, the force applied to these components is cancelled by the reaction force that appears between the mule shoe and these components. In this regard, note that an external diameter of the plug before being set is smaller than an interior diameter of the casing, so that the plug can be moved inside the well at the desired location prior to the setting operation. However, after the plug is set, at least the sealing element has an external diameter that is equal to the internal diameter of the casing.

The mule shoe is assembled to the mandrel last, i.e., it is connected to the mandrel after all of the other components are attached to the mandrel. The connection between the mule shoe and the mandrel must withstand the total setting force. If the connection between the mule shoe and the mandrel fails, the plug also fails to set properly and will not hold pressure, or may even be pumped down the well during the fracture operation.

This failure of the mule shoe may result in fracturing the same stage twice, as all of the fluid will be injected into the previous fracture stage, which is more conductive than the unfractured stage in most cases, and is undesirable.

To prevent this from happening, traditionally, a mule shoe is connected to the mandrel with plural composite pins 218A as illustrated in FIG. 2. Pins 218A are a reliable way to connect the mule shoe to the body of the mandrel. However, this traditional approach is labor intensive as the mule shoe and the mandrel must be match drilled in a jig, and then the pins needs to be inserted. This process is slow and tedious.

One way to make the attachment of the mule shoe to the mandrel faster and less labor intensive is to provide a thread onto an external portion of the mandrel and also onto an internal portion of the mule shoe and to simply attach the mule shoe via the threads to the mandrel. This configuration is shown in FIG. 3, which corresponds to FIG. 14B of U.S. Pat. No. 9,010,411 (herein “the '411 patent”). The plug 300 in FIG. 3 shows the mandrel 310 having a bore 312 and a sleeve 314 being attached to the end of the mandrel. Sleeve 314 has internal round threads 314A and mandrel 310 has external round threads 310A. The two round threads 314A and 310A are manufactured to fit each other. However, a rounded thread is not appropriate for molded components.

Thus, it is desirable to attach the mule shoe onto the mandrel in a more efficient way that overcomes the above noted problems.

SUMMARY

According to an embodiment, there is a composite plug for sealing a well, the composite plug including a mandrel having an internal bore that extends along a longitudinal axis (X) of the mandrel, and plural elements located on the mandrel. At least one element of the plural elements is attached to the mandrel with a tunable thread, and the mandrel and the tunable thread are made of composite materials.

According to another embodiment, there is a method for using a composite plug in a well for sealing the well, the method including lowering the composite plug into the well, wherein the composite plug includes a mule shoe attached to a mandrel with only a tunable thread, sending a signal to a setting tool to deploy the composite plug, and pulling out of the well the setting tool by applying a tensile load to the setting tool so that a connection between the setting tool and the plug is broken while the tunable thread holds the mule shoe attached to the mandrel.

According to yet another embodiment, there is a method for manufacturing a composite plug for sealing a well, the method including providing a loading direction of a mule shoe of the composite plug, wherein the mule shoe is attached with only a tunable thread to a mandrel of the plug; providing a maximum value of a force that the mule shoe needs to withstand while in the well; providing a value for an angle between (1) a steep face of a tooth of the tunable buttress thread and (2) a longitudinal axis of the mandrel; selecting a method for making the mule shoe from a composite material; and making the mule shoe to have the tunable thread with the given angle.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

FIG. 1 illustrates a well and associated equipment for well completion operations;

FIG. 2 illustrates a traditional composite plug;

FIG. 3 illustrates a composite plug having a sleeve that is attached with threads and pins to a mandrel;

FIG. 4A illustrates a round thread made with filament winding, FIG. 4B illustrates a v-shaped thread made with filament winding, FIG. 4C illustrates a round thread made with a direct mold method, and FIG. 4D illustrates a v-shaped thread made with the direct mold method;

FIG. 5A illustrates a buttress thread made with the direct mold method, FIG. 5B illustrates a reverse buttress thread made with the direct mold method, FIG. 5C illustrates a tooth of a buttress thread; and FIG. 5D illustrates various parameters of a buttress thread;

FIG. 6 illustrates a plug having a mule shoe attached with a buttress thread to a mandrel;

FIG. 7 illustrates a plug having a mule shoe and another element attached with corresponding buttress threads to a mandrel;

FIG. 8 illustrates a plug having two elements attached with corresponding buttress threads to a mandrel;

FIG. 9 illustrates a plug having two elements attached with a buttress thread and a non-buttress tread, respectively, to a mandrel;

FIG. 10 is a flowchart for a method of using a plug in which at least one element is attached to a mandrel with a buttress thread; and

FIG. 11 is a flowchart of a method for manufacturing a plug in which at least one element is attached to a mandrel with a buttress thread.

DETAILED DESCRIPTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to attaching a mule shoe to a mandrel of a composite plug. However, the embodiments discussed herein are applicable to any type of plug (not only composite), and also to other components (not only the mule shoe, which is also called a sleeve) of the plug that need to be attached to a mandrel of the plug.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

As discussed above, a mule shoe may be attached to a mandrel with a combination of threads and pins. The threads promote faster attachment of the mule shoe to the mandrel, but they exhibit low shear resistance. For this reason, the existing downhole tools need to also use the pins when attaching the mule shoe to the mandrel.

In the following, mechanisms that explain why the traditional round or v-shaped threads fail under stress are discussed and these teachings are applied to develop new threads, that can withstand the high forces that appear during the setting operation of the plug. These new threads, when designed/selected based on (1) the type of element (e.g., mule shoe or slip ring) to which they are applied, (2) the method of manufacturing of the threads, and (3) the type of forces that would be applied to the threads, would provide enough stress distribution so that the forces associated with the setting of the plug would not break them and also, would make the use of the pins unnecessary. Having a plug for which a mule shoe or other parts are attached to the mandrel without pins would make the assembly of the plug faster and less labor intensive.

To fully appreciate the stress distribution in the new threads of the plug, it is noted that most plugs also contain a feature at the top end (where the plug is connected to the setting tool), which is intended to shear before the mule shoe fails. This feature can be a shear ring, or a set of shear pins. The shear feature is designed to shear and release the plug from the setting tool at the optimum setting force. The strength of the mule shoe connection needs to be greater than the shear force applied to the shear feature or otherwise the plug cannot be set.

The mandrel and/or the mule shoe of a composite plug may be made of composite materials. Because some composite materials show strong stress resistance (the term “stress resistance” is used herein to describe the capability of a material to withstand a certain amount of stress; the higher the stress resistance, the higher the shear force the material can withstand) along one direction and weak stress resistance another direction, a sleeve made from a composite material may not be appropriate for use of round threads.

To understand the stress distribution in a composite material, the method of fabricating the composite material needs to be discussed. Composite materials can be constructed in different ways. For example, if filament wound is used (or wrapped construction), the resultant product, which is usually a tube, like the mandrel in FIG. 2, is strong in hoop strength (i.e., radially), but weak in shear (i.e., along the long direction of the wrapped tube). The shear force acts between layers of wrapped fibers, while stress in the hoop direction acts perpendicular to the strengthening fibers.

Direct molded parts from chopped or long fiber reinforcement material is stronger in shear than wrapped mandrels, but weaker in hoop strength. A direct molded process uses a given mold, in which fiber composite material having various lengths and/or orientations are placed together with a heat sensitive material. Heat is applied to the mold for a given time and then the mold is cooled. The resulting product is a direct molded part.

FIGS. 4A-4C show round threads 400 and 420 (similar to the one used in the '411 patent discussed above) and FIGS. 4B and 4D show v-shaped threads 410 and 430. The round thread 400 has a smooth shape surface 402 while the v-shaped thread 410 has a sharp shape surface 412. Both threads 400 and 410 were made using filament winding and FIGS. 4A and 4B show the corresponding filaments 404 and 414, in cross-section. This means that these filaments are wound around the structure and they exhibit high resistance to hoop stress, but low resistance to shear stress. Arrow HS indicates the direction of the force associated with the hoop stress while arrow SS indicates the direction of the shear force.

In both cases, the shear force F is large, i.e., the shear force is approximately the force applied by the setting tool to the plug. However, the shear resistance of the threads is week, as the shear force is pulling the filaments away from each other. Thus, with such a configuration of the threads, a mule shoe cannot withstand large shear forces (which are typical in a downhole tool). The arrangement shown in FIG. 3 (associated with the '411 patent) uses the round threads to rotate the stress into the part, decreasing shear and increasing hoop stress.

FIG. 4C shows a round thread 420 and FIG. 4D shows a v-shaped thread 430 that are manufactured using the direct mold method. In this case, the filaments (fibers) 424 and 434 have a random direction throughout the threads. Thus, when a shear force F is applied to these threads, some of the filaments are aligned with the force and some of them are perpendicular to the force. Those filaments that are perpendicular or substantially perpendicular to the shear force will provide a substantial shear resistance, as the force is trying to break up the filaments. This is very different from the configurations in FIGS. 4A and 4B, where the shear force was trying to move apart one filament from another filament. Recognizing that breaking a filament into pieces (as in FIGS. 4C and 4D) is much harder than separating two filaments (as in FIGS. 4A and 4B), it follows that the same round and v-shaped threads, when manufactured with a direct mold method, exhibit increased shear resistance for shear forces that are typically for the plug.

However, even the threads of FIGS. 4C and 4D might not have enough shear resistance to withstand the large shear forces present in the downhole tools.

Thus, the inventors of this application have manufactured one or more elements of the composite plug to have a tunable thread (e.g., a buttress thread) instead of a round or v-shaped thread. A tunable thread is defined herein as a thread, different from a v-shaped thread or round thread, for which the shape of the teeth is calculated based on various requirements, for example, a force that the element in the composite plug needs to withstand in the well. In this regard, the force can be a high force, for which case the tunable thread needs to be stronger (e.g., buttress thread) than a v-shaped thread, or a low force, which case the tunable thread needs to be softer than the v-shaped thread. Such a tunable thread may include the buttress thread, an acme thread, a stub acme, or a tapered thread. The buttress thread is used for simplicity in the following embodiments as an example of a tunable thread. However, one skilled in the art, after reading this disclosure, would understand that any tunable thread may be used. As noted above, the tunable thread excludes the v-shaped thread and the round thread.

A buttress thread 500 is shown in FIG. 5A and a reverse buttress thread 510 is shown in FIG. 5B. For context and not as a limitation, the buttress thread 500 is part of a mule shoe 518 and the mating thread is shown as being part of a mandrel 504. However, as discussed later, these threads may be used between other parts of the composite plug.

Both the buttress thread 500 and the reverse buttress thread 510 are shown having filaments 502 randomly distributed, i.e., they are formed with a direct mold method. If the shear force F is applied as illustrated in FIGS. 5A and 5B, these two configurations benefit not only from the random distribution of the filaments inside the thread, but also from the specific geometry of the threads. In this respect, note that a force that appears between the threads of the mandrel and the threads of the mule shoe in this case is applied to the steep faces of the corresponding teeth, and the slanted face of each tooth provides extra strength to the steep face, to withstand a larger shear force than a round or v-shaped thread.

The force configuration shown in FIGS. 5A and 5B corresponds to the case when the mandrel is pulled and the push ring 206 in FIG. 2, is pushing the slip rings and the wedges toward the mule shoe 218. Note that the forces shown in FIGS. 5A and 5B can be reversed, in which case the shear resistance decreases due to the slanted orientation of the teeth. Thus, orientation of the shear forces relative to the buttress threads is a factor to be considered when designing the components of the plug tool.

In other words, the buttress thread 500 may put the composite member 518 in linear shear (completely) if used in the normal forward direction (i.e., if the shear force F1 shown in FIG. 5C is applied to the steep face 500A), and in complete hoop if used in the reverse direction (i.e., the shear force F2 also shown in FIG. 5C is applied to the slanted face 500B). A reverse direction thread can be constructed at any given angle to “tune” the stress direction in a frac plug and balance between hoop and shear stress for maximum strength. It can also be used in mostly shear or mostly hoop to intentionally break a composite part made using wrapped method or molding method (respectively) at a planned loading. Therefore, a tunable buttress thread can be used in multiple places on a tool to have intentional weak points. It could also be used in conjunction with a standard v-thread, and be either weaker or stronger, depending on the material and design of the thread. Thus, the buttress thread can be “tuned” depending on the needs, to have a desired mixture of hoop and shear stress, depending on the situation which the component having the buttress stress is facing.

More specifically, considering the angle between the steep face 500A and the longitudinal axis X of the element (mule shoe in this case) that has the thread, to be α and the angle between the slanted face 500B and the longitudinal axis X to be β, angle α can extend between about 90° (i.e., substantially perpendicular to the X axis as shown in the figures) to less than 135° (which is the angle corresponding to the v-shaped thread). Angle β can extend from about 90° to less than 180°.

If the force applied to the thread is F1 as illustrated in FIG. 5C, and it is desired to obtain maximum stress resistance, then angle α should be about 90, the thread should be buttress, and the manufacturing method should be direct mold (thus random distribution of filaments 502 in each tooth). If wound filament is used to make the teeth, the shear resistance is diminished. Also, if the angle is increased toward 135, the shear resistance is further diminished. For the reverse buttress thread, if angle β is about 180, the force F2 is applied as shown in FIG. 5C (note that only the component F_(2⊥) perpendicular to the slanted face 500B acts on the teeth), and the manufacturing method for making the teeth is filament wound, then the teeth are in full hoop. If the angle is decreased towards 90 and the manufacturing method for making the teeth is direct mold, the teeth exhibit maximum shear resistance. One skilled in the art would note that angles α and β are related to each other in the sense that when one increases, the other decreases and vice versa, and thus, one is dependent on the other one. Thus, the parameters that characterize the shear resistance of buttress threads are: the angle of the steep or slanted face, the method of manufacturing the teeth, and the direction of the applied force. By choosing one specific combination of these three parameters, a buttress thread able to withstand a given shear force may be achieved.

Further, it is noted that a shape of each tooth may also impact the properties of the buttress thread. For example, FIG. 5D shows a distance between two adjacent teeth is the pitch P, a height H of a tooth is measured from a root 522 to a crest 524, the crest 524 may be flat and may have a width W. All these additional parameters may also be selected for each element of the plug or another downhole tool to achieve a predetermined shear resistance. In one application, it is possible to combine two or more thread configurations for different parts of the plug, one type of thread for the mule shoe and another one for a slip ring.

An example would be to use a strong buttress thread connection at the mule shoe (i.e., normal buttress thread with molded mandrel and molded mule shoe), and a weak buttress thread at the top of the plug (wrapped shear member, normal thread). These thread forms could be used at other parts of the plug, such as under slip rings or under wedges, to change the timing and break points of the plug setting.

The buttress threads discussed above are shown in FIG. 6 as being used between a mule shoe 618 and a mandrel 604 for a plug 600. Mandrel 604 has a bore 602 (which extends along a longitudinal axis X of the mandrel) and the following elements are placed on the mandrel: push ring 606, upper slip ring 608, upper wedge 610, sealing element 612, lower wedge 614, lower slip ring 616, and mule shoe 618. FIG. 6 shows a buttress thread 618A formed on the inside region of the mule shoe 618 and a corresponding buttress thread 604A formed on an outside region of the mandrel 604. In this embodiment, both the mandrel and the mule shoe are made of the same or different composite materials. It is noted that no pins or other devices are used for securing the mule shoe to the mandrel in this embodiment. The threads 604A and 618A are the only mechanism for attaching the mule shoe to the mandrel.

FIG. 7 shows another embodiment in which two different elements of a plug 700 are attached with buttress threads to the mandrel. The first element is the mule shoe 618, which is attached with a first buttress thread 618A to a first buttress thread 604A of the mandrel 604, and the second element is the upper slip ring 608, which is attached with a second buttress thread 608A to a corresponding second buttress thread 604B of the mandrel. Thus, in this embodiment, the first buttress thread 604A of the mandrel may be different from the second buttress thread 604B, at least in one of: the angle α between the steep face and the longitudinal axis X of the mandrel, the angle β between the slanted face and the longitudinal axis X of the mandrel, the pitch P between the teeth, the height H of the teeth, the form of the teeth (as discussed in FIG. 5D), or an external diameter of the mandrel at the location of buttress thread. One skilled in the art would understand that a combination of two or more of these parameters may be used to achieve the different first and second buttress threads 604A and 604B. One skill in the would also understand that the first element may be any of the elements of the plug and the second element may be any of the elements of the plug. In other words, it is not necessary that one element of the plug is always the mule shoe.

In one embodiment, the first and second elements are the upper and lower slip rings 608 and 616 as illustrated in FIG. 8. Thus, mandrel 604 has first and second buttress threads 604A and 604B, that mate with first buttress thread 616A of lower slip ring 616 and second buttress thread 608A of upper slip ring 608, respectively. In this embodiment, the mule shoe is attached with pins to the mandrel as in the traditional case. However, it is possible that no pins are used for the mule shoe and a third buttress thread is provided in the mandrel and the mule shoe for attaching these two elements to each other.

In one further embodiment, as illustrated in FIG. 9, a first element of a plug 900 is attached to the mandrel with a buttress thread and a second element of the plug is attached to the mandrel with a traditional (round or v-shaped) thread. FIG. 9 shows plug 900 having the mule shoe 618 attached with a buttress thread 618A to a corresponding buttress thread 604A formed in the mandrel 604, and also having the upper slip ring 608 attached with a round or v-shaped thread 608A to a corresponding round or v-shaped thread 604B, formed into the mandrel. Those skilled in the art would understand that the first and second elements of the plug may be any of the elements shown in the figures. Note that each embodiment in which buttress treads are used for attaching the mule shoe to the mandrel is free of pins or other mechanism for maintaining the mule shoe in place.

A method of manufacturing a buttress thread for one component of a composite plug is discussed with regard to FIG. 10. For simplicity, the component is considered herein to be the mule shoe. However, the component may also be the slip ring or the wedge. In step 1000, the loading direction of the mule shoe is provided. The loading direction provides information about the direction of the shear force that will be applied to the mule shoe. In step 1002, the maximum value of the force that the mule shoe needs to withstand is provided. In step 1004, the angle between the steep face and a longitudinal axis of the mule shoe is provided. In step 1006, the method for making the mule shoe is provided, i.e., filament winding or direct mold. In step 1008, the mule shoe is manufactured to have an internal buttress thread with the angle between the steep face and the longitudinal axis of the mule shoe provided in step 1004. The number of threads made in the mule shoe and the angle account for the amount of force that the mule shoe will withstand. The features of each tooth may also be selected before making the buttress thread.

A method of using a plug with at least one buttress thread is now discussed with regard to FIG. 11. In step 1100, the plug having at least one component attached only with a buttress thread to the mandrel is lowered into the well to a desired depth. For simplicity, the at least one component is considered to be the mule shoe. A fluid may be pumped from above to move the plug to the desired location. In step 1102, a signal is sent along a wireline, which is connected to a setting tool, to deploy the plug. In this step, the setting tool pulls on the mandrel and pushes on the various components of the plug until a sealing element of the plug extends radially and seals the well. In step 1104, the setting tool is pulled outside the well by applying a tensile load, so that a connection between the setting tool and the plug is broken, which releases the setting tool. In an additional step, the applied tensile load is smaller than the maximum force at which the teeth of the buttress thread are sheared. In one step, the maximum force is calculated to be larger than a maximum force that breaks the connection between the setting tool the plug. In an optional step, no pins or other connecting mechanisms, except the buttress thread, is used to attach the mule shoe to the mandrel. In still another optional step, one or more other elements of the plug are attached to the mandrel with another buttress thread. The another buttress thread may be different from the original buttress thread between the mandrel and the mule shoe. In yet another optional step, the another thread is a round or v-shaped thread.

The disclosed embodiments provide methods and systems for attaching an element, e.g., a mule shoe, to a mandrel of a plug with only a buttress thread. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. 

What is claimed is:
 1. A composite plug for sealing a well, the composite plug comprising: a mandrel having an internal bore that extends along a longitudinal axis (X) of the mandrel; and plural elements located on the mandrel, wherein at least one element of the plural elements is attached to the mandrel with a tunable thread, wherein the mandrel and the tunable thread are made of composite materials.
 2. The composite plug of claim 1, wherein the at least one element is one of a mule shoe, an upper slip ring, an upper wedge, a sealing element, a lower wedge, and a lower slip ring.
 3. The composite plug of claim 1, wherein the tunable thread is a buttress thread.
 4. The composite plug of claim 1, wherein the at least one element is attached to the mandrel only with the tunable thread.
 5. The composite plug of claim 1, wherein at least another element of the plural elements is attached to the mandrel with another tunable thread.
 6. The composite plug of claim 5, wherein the at least one element is a mule shoe and the at least another element of the plural elements is one of an upper slip ring, an upper wedge, a sealing element, a lower wedge, and a lower slip ring.
 7. The composite plug of claim 5, wherein the tunable thread is different from the another tunable thread.
 8. The composite plug of claim 5, wherein the tunable thread is manufactured with a direct mold method and the another tunable thread is manufactured with a filament wound method.
 9. The composite plug of claim 1, wherein the tunable thread is manufactured with a direct mold method.
 10. The composite plug of claim 1, wherein the tunable thread is a buttress thread and a tooth of the tunable buttress thread has a steep face and a slanted face, and an angle of the steep face with the longitudinal axis of the mandrel is selected to be larger than
 90. 11. A method for using a composite plug in a well for sealing the well, the method comprising: lowering the composite plug into the well, wherein the composite plug includes a mule shoe attached to a mandrel with only a tunable thread; sending a signal to a setting tool to deploy the composite plug; and pulling out of the well the setting tool by applying a tensile load to the setting tool so that a connection between the setting tool and the plug is broken while the tunable thread holds the mule shoe attached to the mandrel.
 12. The method of claim 11, wherein the mandrel has an internal bore that extends along a longitudinal axis (X) of the mandrel and plural elements are located on the mandrel, and wherein the mandrel and the tunable thread are made of composite materials.
 13. The method of claim 11, wherein the tunable thread is manufactured with a direct mold method.
 14. The method of claim 11, further comprising: applying the tensile load to the setting tool to be smaller than a maximum force at which teeth of the tunable thread are sheared.
 15. The method of claim 11, wherein the tunable thread is a buttress thread.
 16. A method for manufacturing a composite plug for sealing a well, the method comprising: providing a loading direction of a mule shoe of the composite plug, wherein the mule shoe is attached with only a tunable thread to a mandrel of the plug; providing a maximum value of a force that the mule shoe needs to withstand while in the well; providing a value for an angle between (1) a steep face of a tooth of the tunable buttress thread and (2) a longitudinal axis of the mandrel; selecting a method for making the mule shoe from a composite material; and making the mule shoe to have the tunable thread with the given angle.
 17. The method of claim 16, wherein the method for making the mule shoe is a direct mold method.
 18. The method of claim 16, further comprising: making the mandrel to have a mating tunable thread that mates with the tunable thread of the mule shoe.
 19. The method of claim 16, further comprising: attaching another element of the plug to the mandrel with another thread.
 20. The method of claim 19, wherein the another element is one of an upper slip ring, an upper wedge, a sealing element, a lower wedge, and a lower slip ring.
 21. The method of claim 19, wherein the another thread is another tunable thread.
 22. The method of claim 21, wherein the another tunable thread is different from the tunable thread.
 23. The method of claim 19, wherein the another thread is a round or v-shaped thread.
 24. The method of claim 19, wherein the tunable thread is manufactured with a direct mold method and the another thread is manufactured with a filament wound method.
 25. The method of claim 16, wherein the tunable thread is a buttress thread. 